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Major and minor grooves

The strand backbones are closer together on one side of the
helix than on the other. The major groove occurs where the
backbones are far apart, the minor groove occurs where they are
close together. The grooves twist around the molecule on opposite
sides. Certain proteins bind to DNA to alter its structure or to
regulate transcription (copying DNA to RNA) or replication (copying DNA
to DNA). It is easier for these DNA
binding proteins to interact with the
bases (the internal parts of the DNA molecule) on the major groove
side because the backbones are not in the way.